1. Field of the Invention
This invention relates to a method of and material produced by electrolysis of quartz in a vacuum. Specifically, this invention relates to a method of treating crystal resonator quartz rendering it insensitive to transient frequency offset due to bursts of ionizing radiation.
2. Description of the Prior Art
Accumulated performance data on the stability of quartz resonators in earth orbiting satellites, as well as similar information gained from deep space probes, underscore the importance of reducing the effects of ionizing radiation on these critical system components.
Prior to the last decade, studies of the effect of ionizing radiation on crystal resonators dealt exclusively with the change in frequency resulting from exposure to continuous or steady state radiation. In general, natural quartz resonators, such as 5th overtone 5-MHz crystal units, exhibit a change in frequency withh irradiation. For example, optical grade natural Brazilian quartz resonators exhibit a negative change in frequency with irradiation, as much as 10 to 50 parts per million (ppm) on exposure to 10.sup.6 rad. Conversely, electronic Z-growth synthetic quartz resonators usually exhibit a positive offset of less than 10 ppm on exposure to 10.sup.6 rad.
Swept quartz is relatively unaffected by steady-state ionizing radiation. The term swept quartz refers to quartz treated by a method described in U.S. Pat. No. 3,113,224 to J. C. King on Dec. 3, 1963. This method generally comprises subjecting a quartz crystal in air to an electric field of at least 500 V/cm at a temperature of at least 400.degree.C for a period of at least 12 hours. Such air-swept quartz in resonators commonly yields frequency offsets of no greater than 0.1 ppm after a cumulative exposure of 10.sup.6 to 10.sup.7 rad.
Thus means have been found to process quartz to render its use in a resonator relatively immune to steady-state radiation. However, it is also extremely important to assure that resonators are not adversely affected by pulses of radiation. On exposure to a pulse of ionizing radiation, it has been observed that virtually all quartz, whether natural, synthetic or swept synthetic, experiences a transient negative frequency shift which anneals within a few minutes after the pulse toward a relatively stable offset.
Following exposure to a 70-nanosecond X-ray pulse, natural quartz resonators approach a stable offset in 100 to 1,000 seconds. The post-irradiation stable resonant frequency is lower than its pre-pulse radiation value. Unswept synthetic quartz also experiences a transient negative frequency shift and the annealing continues for several hours until a stable positive offset results. While the transient offset for swept synthetic crystal is virtually identical to unswept and natural material, it anneals substantially to its pre-irradiation frequency.
In addition to air swept electrolysis of quartz, researchers have experimented with vacuum sweeping of quartz for a variety of purposes. U.S. Pat. No. 3,337,439 issued Aug. 22, 1967, to D. B. Fraser discloses vacuum sweeping of quartz to reduce acoustic absorption of high Q synthetic quartz at room temperature. U.S.S.R. researchers L. G. Chentsova, et al. in Kristallografiya, 17, 374-378 (1972) Eng. Transl. [Soviet Phy. (Cryst.) 17, 317 (1972)], report that A-band color centers can be induced in quartz by subjecting it to vacuum electrolysis at a temperature where the quartz is in the beta phase; i.e., above 573.degree.C.
The prior art has been silent as to the theoretical aspects or a practical means for treating quartz to assure that after pulse irradiation, a quartz resonator will experience an acceptably small frequency offset.
It is an object of this invention to provide methods of treating quartz useful in resonator circuits so that when subjected to pulse irradiation, virtually no transient frequency offset occurs.